The role of the thymus in cell-mediated immunity was discovered about 50 years ago and it has since become apparent that the thymus is the site of T cell development in all extant vertebrates. Although it has been recognized that thymopoiesis can only be fully understood by the analysis of both the thymic epithelial microenvironment and the intrathymic haematopoietic compartment, significant gaps in our knowledge of thymopoiesis remain.The proposal addresses major biological questions of thymopoiesis in a novel way. It is based on an evolutionarily informed approach that makes iterative use of distinct animal models (fish and mouse), and additionally considers information obtained from the analysis of human patients with thymopoietic deficiencies. The proposal aims to develop an animal model conducive to transplantation of xenogeneic haematopoietic progenitor cells. This system will be used for subsequent analysis of haematopoietic and immunological properties of genetically intractable fish species. The proposal also aims to determine the structure of genetic networks underlying the development and function of thymic epithelial cells and intrathymic T cell differentiation. This information will be used to explore the possibility of alleviating the phenotypic consequences of disease genes by interfering with the function of genes situated elsewhere in the respective genetic networks rather than by restoring faulty gene function. Evolutionarily constrained inventories of such genetic networks will be additionally used for the design of synthetic thymopoietic environments in non-cognate tissues, either using combinations of thymopoietic factors or by exploiting the thymopoietic capacity of Foxn1, a master regulator of thymic epithelial cell differentiation. Overall, the proposal aims at the development of evolutionarily informed genetic and cell-based strategies to reverse failing thymus function.